ON THE TOPOLOGICAL STABILITY OF ASTROPHYSICAL JETS

General theoretical arguments and various analytic self-similar soluti ons have recently shown that magnetized and rotating astrophysical out flows may become asymptotically cylindrical, in agreement with observa tions of cosmical jets. A notable common feature in all such self-cons istent, self-similar MHD solutions is that before final cylindrical co llimation is achieved, the jet passes from a stage of oscillations in its radius, Mach number and other physical parameters. It is shown tha t under rather general assumptions this oscillatory behaviour of colli mated outflows is not restricted to the few specific models examined s o far, but instead seems to be a rather general physical property of a n MHD outflow that starts non-cylindrically before it reaches collimat ion. It is concluded thence that astrophysical jets are topologically stable to small-amplitude, time-independent perturbations in their asy mptotically cylindrical shape. Also, similarly to the familiar fluid i nstabilities, these oscillations may give rise to brightness enhanceme nts along jets.